This invention relates to a kinescope pincushion distortion correction circuit.
It is known in the art that side or East-West pincushion distortion of the raster on a kinescope such as utilized in a television receiver may be substantially eliminated by modulating the horizontal rate deflection current amplitude through the horizontal deflection coils by a substantially parabolic current component at a vertical scanning rate. Generally the desired modulation has been accomplished by passive currents in which a control or primary winding of a saturable reactor or transformer is energized by vertical rate energy and a secondary winding is placed in circuit with the horizontal deflection winding. The horizontal deflection current amplitude is modulated by the vertical deflection current such that the raster width is reduced at the top and bottom of the raster.
Another known arrangement for side pincushion distortion correction involves a capacitor coupled in parallel with the vertical deflection winding. As is disclosed in copending application Ser. No. 07161/75 for Peter E. Haferl and entitled "VERTICAL DEFLECTION SYSTEM", the capacitor is charged by energy from the horizontal retrace pulse under the control of switches. In both the passive saturable reactor circuits and in the switched vertical deflection circuit according to the aforementioned copending application, side pincushion correction is obtained by loading the high voltage transformer of the horizontal deflection system during the horizontal retrace time. In order to obtain correctly shaped side pincushion correction the loading of the high voltage transformer is modulated at the vertical deflection rate, as by the vertical deflection current. Thus, maximum loading occurs at the top and bottom of the picture and minimum loading occurs at the center of the picture.
The variable loading of the horizontal retrace pulse at the vertical rate results in the generation of a further pincushion distortion, known as inside pincushion distortion to distinguish from the outside or peripheral pincushion distortion ordinarily referred to. This further pincushion distortion occurs within the raster as a result of time modulation of the start of horizontal scan caused by the vertical rate loading. Increased trace duration resulting from time modulation of the horizontal retrace pulse at the top and bottom of vertical scan increases the portion of the resonant period of the deflection coil 26 with S correction capacitor 28 subtended during trace. Thus, the inside pincushion distortion appears in the region between the center line and the extreme left and right sides of the picture as an insufficient pincushion correction.
The amount of inside pincushion correction depends upon the geometry of the picture tube and on the amount of outside pincushion distortion requiring correction. With the advent of wide-angle large viewing screen picture tubes it has been found that the inside pincushion distortion may be objectionable to the point that correction is required.
A prior art arrangement for the solution of the inside pincushion correction problem, in addition to structure utilized for conventional pincushion correction, uses a separate saturable reactor or transductor in series with the horizontal deflection winding. The control winding of the saturable reactor is driven by a vertical deflection rate signal and modulates the inductance of the horizontal deflection circuit to correct for the change in "S" shaping and thereby correct the inside pincushion distortion. This prior art solution has disadvantages which include critical design of the saturable reactor, temperature dependence of the saturable reactor, cost of the saturable reactor, and a control range so limited as to often be insufficient to compensate for construction tolerances.